Touch screens are prevalent in today's computing environment. Portable computers, desktop computers, tablets, smart phones, and smartwatches typically employ a touch screen to gain user input for navigation and control of these devices. Thus, discerning the intent of the user via touch inputs is an important feature of a touch screen device.
Touch screens typically operate based on capacitive touch sensing, and include a patterned array of conductive features. For instance, the patterned array of conductive features may include sets of lines, conductive pads, overlapping structures, interleaved structures, diamond structures, lattice structures, and the like. The conductive features form capacitive nodes at various points. By evaluating changes in capacitance at each capacitive node, a user touch or hover, such as by a finger or, can be detected. Such a touch results in a touch data island when the patterned array of conductive features is read. A touch data island is a group of interconnected nodes with strength values having a magnitude above a certain threshold.
It is common for conventional touch sensing techniques to support detection of multiple touches. Finger separation algorithms are commonly applied to each touch data island so as to determine the presence of multiple simultaneous touches. Conventional finger separation algorithms are more effective under normal touch sensing modes than under conventional floating touch sensing modes.
Shown in FIG. 1A is a first operating condition in which the user is holding the touch screen device in their hand while touching the touch screen with their finger. The ground level of the touch screen device and the finger are the same, and the capacitances between the touch screen device and ground (shown as Cphone) and between the finger and ground (shown as Cfinger) are the same. In this condition, conventional finger separation algorithms are effective.
Shown in FIG. 1B is a second operating condition in which the touch screen device is plugged into a power cord to receive power from electrical mains while the user is touching the touch screen with their finger, but is not holding the touch screen device. The ground level of the touch screen device and the finger are the same, and the capacitances between the touch screen device and ground (shown as Cphone) and between the finger and ground (shown as Cfinger) are the same. In this condition, conventional finger separation algorithms are effective.
Shown in FIG. 1C is a third operating condition in which the user is not holding the touch screen device in their hand (and the touch screen device is resting on a non-conductive surface) but the user is touching the touch screen with their finger. Here, the ground level of the touch screen device and the finger are not the same, and the capacitances between the touch screen device and ground (shown as Cphone) and between the finger and ground (shown as Cfinger) are not the same. In this third operating condition, which can be referred to as a floating condition, conventional finger separation algorithms may not be effective, and may falsely report multiple touches in a touch island when there was actually only a single touch
The touch screen device in FIG. 2 is operating in the third operating condition, and it can be seen that the user is touching the touch screen. This touch screen device is executing a conventional finger separation algorithm which is incorrectly interpreting the touch by one large finger (a thumb) to be two touches C1 and C2.
A chart showing touch data values from the group of interconnected nodes (e.g., touch sensors) of the touch screen during the touch of FIG. 2, in the third operating condition, is shown in FIG. 3. Here, it can be seen that in the touch island 1 there are three circled touch areas 2 with large negative touch data (e.g., strength) values, which will result in unreliable interpretation of the touch. Here, multiple touches will be registered from the touch island, yet only one touch should be registered as only one finger is touching the touch screen.
Therefore, new operating techniques are needed so that the touch screen device can properly differentiate single touches from multiple touches in the floating condition.